In recent years, it has been considered to apply a modular multilevel converter (MMC) to a self-excited reactive power compensation device such as a static synchronous compensator (STATCOM), a direct-current (DC) power transmission system such as a back to back (BTB) system, a motor drive inverter, and the like (for example, see WO2012/099176 (PTL 1)).
The MMC is configured with a plurality of unit converters connected in series. Each unit converter includes a main circuit that has switching elements and a DC capacitor. The unit converter causes the switching elements of the main circuit to perform switching operation to thereby output a voltage of the DC capacitor to output terminals. As a switching element, a semiconductor switching element that can be subjected to on/off control, such as an insulated gate bipolar transistor (IGBT), is generally used.
The main circuit further has a bypass switch connected between a pair of output terminals. The bypass switch is configured to be turned on when an abnormality such as a short circuit of a switching element occurs, to establish a short circuit between the pair of output terminals of a unit converter in which the abnormality occurs.